1. Field of the Invention
The present invention relates to a fuel cell stack wherein a plurality of unit cells are stacked with interconnectors interposed therebetween and, more particularly, to a fuel cell stack wherein cooling pipes for circulating a coolant are embedded in at least one of the interconnectors.
2. Description of the Prior Art
A fuel cell is conventionally known as a power generator to obtain direct current power by the electrochemical reaction between a gas which is easily oxidized, such as hydrogen, and a gas which has an oxidizing ability, such as oxygen. In a fuel cell of this type, a matrix containing an electrolyte solution is generally interposed between a pair of gas diffusion electrodes. The outer surface of one electrode is brought into contact with a gas (fuel) containing hydrogen, while the outer surface of the other electrode is brought into contact with a gas (oxidizing agent) containing oxygen, with a load connected between both electrodes. Then, direct current power is supplied to the load. A catalyst layer carrying platinum or the like is generally formed on each of the gas diffusion electrodes so as to facilitate the reaction. A power generator is known which comprises a plurality of series-connected unit cells, each unit cell comprising a fuel cell as described above.
The fuel cells which are series-connected as unit cells generally have a configuration as shown in FIG. 1. More specifically, in a unit cell 4, a matrix 3 containing an electrolyte solution is interposed between a pair of gas diffusion electrodes 2a and 2b having catalyst layers 1a and 1b formed on their inner surfaces. Such unit cells 4 are stacked with conductive interconnectors 5 comprising carbon plates or the like interposed therebetween. Grooves 6 for passing the fuel gas therealong are formed on one surface of each interconnector 5 to extend in the direction indicated by arrow P. Grooves 7 for passing the oxidizing gas therealong are formed on the other surface of each interconnector 5 to extend in the direction indicated by arrow Q perpendicular to that indicated by arrow P. In some interconnectors 5, for example, in one of every three interconnectors 5, cooling pipes 8 are embedded for preventing a temperature rise in the cell due to heat generated by the electromotive reaction.
An interconnector 5 with the cooling pipes 8 embedded therein generally has a configuration as shown in FIG. 2. Interconnector mates 11 and 12 are separately molded to have grooves 7 and 6 respectively on one surface of each thereof. The interconnector mates 11 and 12 are adhered together with a conductive adhesive resin such that the grooves 7 and 6 may face outward and be perpendicular to each other. A plurality of grooves 13 are formed on the other surface of the interconnector mate 11 which faces the interconnector mate 12. A plurality of U-shaped cooling pipes 8 coated with insulating films on their outer surfaces are embedded in a sealant 14 within the grooves 13.
A fuel cell stack can be cooled by incorporating such interconnectors having cooling pipes embedded therein as described above and by circulating a coolant through these cooling pipes. However, such an interconnector has a low cooling effect and fails to improve performance of the fuel cells as will be described below.
The interconnector as described above is prepared by adhering together a pair of interconnector mates 11 and 12 with a conductive adhesive resin. In order to prevent warpage during molding of the interconnector mates 11 and 12, they must have a minimum thickness of 5 mm. Further, the interconnector mate 11 must have grooves for embedding the cooling pipes 8 therein in addition to the grooves 7 for circulating the oxidizing gas. For this reason, the interconnector mate 11 must have a greater thickness than that of the interconnector mate 12 in order to guarantee its mechanical strength. For example, if the cooling pipes 8 have a diameter of 3 mm, the grooves 13 in which they are embedded must have a depth of about 3.5 mm. If the grooves 7 have a depth of 2 mm and the remaining portion of the interconnector mate 11 has a thickness of 2.5 mm, the interconnector mate 11 must have an overall thickness of 8 mm. Then, the overall thickness of the interconnector obtained by adhering the two interconnector mates together becomes 13 mm. However, allowing a safety factor, the thickness of the interconnector must be about 15 mm. If an interconnector has such a thickness, the distance between its surface and the cooling pipes 8 increases, resulting in an increase in heat resistance and a low cooling effect. When the cooling pipes 8 are embedded in the grooves 13 of the interconnector mate 11, a conductive thermosetting adhesive such as a carbonaceous material or epoxy resin is used as a sealant. This type of adhesive has unsatisfactory fluidity and tends to form air voids between the inner surfaces of the grooves 13 and the cooling pipes 8. A number of voids are also formed within the adhesive layer by evaporation of a solvent when the adhesive is thermoset. This decreases the effective heat transfer area and also results in a low cooling effect. According to experiments conducted by the present inventors, when a current of 200 mA/cm.sup.2 density was flowed in a conventional fuel stack incorporating the interconnectors as described above, the temperature of the coolant at the outlet port of the cooling pipe was 170.degree. C. However, a maximum temperature of 220.degree. C. was measured at the surface of the interconnector. A temperature difference of 50.degree. C. thus observed indicates the low cooling effect of the cooling pipes.
As a conductive thermosetting adhesive, only a conductive thermosetting adhesive of epoxy resin type containing silver is currently known. The upper limit of the temperature of such an adhesive is as low as 170.degree. to 180.degree. C. Therefore, if the output current density of the cell increases, various problems are caused including easy separation of the interconnector mates, a decrease in the electric conductivity at the adhered surfaces of the interconnector mates, nonuniform current distribution of the interconnector, or a significant increase in the resistance loss.
Since the interconnector mates have different thicknesses, the resultant interconnector will inevitably warp.